In the midwestern summer, humans and plants have to breathe through the heat and humidity. Researchers hope that a retooled crop plant—one with an improved ventilation system within its leaves—could thrive while avoiding drought stress by reducing "crop sweat". Thanks to a next generation imaging technology powered by Argonne National Laboratory's particle collider beamline, a team of scientists now has precise schematics of the leaf's interior and can refine approaches to breeding hardier crop plants.
The research, published in Plant Physiology , was led by postdoctoral researcher James Fischer in the laboratory of Professor of Plant Biology and Crop Sciences Andrew Leakey at the University of Illinois Urbana-Champaign. Their work represents the first detailed look at how the pores on the leaf surfaces of sorghum, a highly productive and resilient grass crop, connect to the air pathways, photosynthetic centers, and veins beneath.
"There are connections between each component of the leaf . . . it's a highly organized system," Fischer said. "We are really defining the leaf beyond just, carbon dioxide goes in, water comes out."
The motivation for the study came from a fundamental tradeoff that every land plant faces. During photosynthesis, plants take in carbon dioxide through tiny pores in their leaves called stomata. For each molecule of CO2 that enters the plant through the stomata to be captured by photosynthesis, 300-400 water molecules escape from the interior of the leaf through the stomata to the atmosphere. A hot, bright day is good for photosynthesis, but terrible for water loss as corn and other crops "sweat it out." This loss of water is why crops only grow in times and places with adequate rainfall. One of the research team's primary goals is to develop crop plants that handle this tradeoff better.
"We're trying to minimize how many water molecules escape while CO2 is going into the leaf to be captured by photosynthesis," Leakey said. Leakey is also the director of the US Department of Energy-funded Center for Advanced Bioenergy and Bioproducts Innovation, which developed plants with fewer stomata that were examined in the new study. "We've engineered plants that have fewer stomata and demonstrated proof of concept for that goal . . . But now we're really interested in quantifying how easily a CO2 molecule can work its way through the interior ventilation system of the leaf to the location where it actually gets captured by photosynthesis."
To answer this question, the team needed what any engineer needs; schematics of how the system currently works. For most plants, such detailed knowledge of the internal anatomy of a leaf simply doesn't exist. The structures inside can only be truly understood in three dimensions, so traditional two-dimensional images from microscopes are not ideal.
Fischer and Leakey seized the chance to apply a high-resolution three-dimensional method called micro-computed tomography that could reveal the path of air flow through an intact, living sorghum leaf. They formed a collaboration with Yale University plant physiological ecology professor Craig R. Brodersen and Guillaume Théroux-Rancourt, assistant director of the agronomy research company Biopterre. Brodersen and Théroux-Rancourt had previously helped to innovate microCT imaging for plant tissue samples.
MicroCT works on the same principle as the CT scan one might get as part of a medical diagnostic process. A series of x-ray images are taken and then combined to form a three-dimensional view of a structure or tissue. Imaging smaller structures requires stronger and more focused x-ray beams than most CT machines can generate. However, these types of x-rays are emitted as a by-product—an incredibly valuable one—of particle accelerator facilities used for atomic physics research. To image their sorghum samples, the research team obtained time at Argonne National Laboratory's DOE-funded Advanced Photon Source.
Once the images were acquired and assembled, with the help of machine learning, into high-resolution three-dimensional models, the researchers were surprised by what they found when comparing plants with more or fewer stomata.
"We thought it was something that would be a challenge for us from the engineering perspective . . . reducing the number of stomata was going to mean that there's a longer, more tortuous path to get CO2 where it's going," Leakey said.
Instead, they found that in leaves with fewer stomata, air spaces beneath them were larger, resulting in equivalent conductance of CO2 through the leaf, meaning that water loss could be reduced without a parallel loss of CO2 distribution to photosynthetic cells. This is good news for further development of drought-stress avoiding plants.
"There are loads of compensatory mechanisms where we're doing this kind of engineering that make our life difficult," Leakey said. "This was the first one that was making our life easier."
Another surprising result was that stomata on the upper surface were positioned over veins rather than between them, as usually occurs to allow more space for internal air spaces to channel CO2 to photosynthetic cells.
"This finding opened our eyes to how the two leaf surfaces may contribute differently to uptake of CO2 for photosynthesis and cooling of the leaf through water loss," Leakey said. "So we want to know, why is it organized like that and how can we leverage that knowledge to make more water use efficient crops that can avoid drought-induced yield losses? That's the exciting next step."
Fischer and Leakey are considering how future studies could answer these and other questions, bringing crop development closer to water use efficiency. In addition to funding by the DOE, the present work was supported by the Howard and Maryam Newman Plant Science Fund.
